Marketplace - How automatically add custom feature field to products

[Gombi92]

I'm looking for an expert advice..

 

I've bought Advanced Marketplace for Prestashop 1.6.

I've seen the "Prestashop Marketplace Product Combination" module but it doesn't really fit my need..
I'd like to collect a department number during seller registration process and add this field to each new product created by the seller in order the navigation layered to allow such selection.

According to me, this functionality should be natively included in the module but is not...

First, I've created feature "department" values from 1 to 99 in back office. (=France).
Then, I have created a new field called "departement_seller" in "marketplace_seller_info" table (it 's a number between 1 and 99).
This field is filled by the sellers during the registration process.
With my customisation, I can display this field value in front product.tpl using {$departement_seller} and in admin BO as well but
what i'm searching to do is to affect this field value to every products added by sellers in order to allow blockfeatured navigaton to display those selection.

I presume I should add some code to addproductprocess.php but don't know how to do that.
I would really appreciate your help,

Thanks,